Method of performing random access procedure in wireless communication system

ABSTRACT

A method includes transmitting a random access preamble, receiving a random access response as a response of the random access preamble, wherein the random access response comprises an uplink resource assignment and a request for transmission of a Channel Quality Indicator (CQI), and transmitting the CQI in the uplink resource assignment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. Provisionalapplication Ser. No. 61/020,399 filed on Jan. 11, 2008, Korean PatentApplication No. 10-2008-0001293 filed on Jan. 4, 2008, and Korean PatentApplication No. 10-2008-0033253 filed on Apr. 10, 2008 which areincorporated by reference in its entirety herein.

BACKGROUND

1. Technical Field

The present invention relates to wireless communications, and moreparticularly, to a method of performing a random access procedure in awireless communication system.

2. Related Art

Third generation partnership project (3GPP) mobile communication systemsbased on a wideband code division multiple access (WCDMA) radio accesstechnology are widely spread all over the world. High-speed downlinkpacket access (HSDPA) that can be defined as a first evolutionary stageof WCDMA provides 3GPP with a radio access technique that is highlycompetitive in the mid-term future. However, since requirements andexpectations of users and service providers are continuously increasedand developments of competing radio access techniques are continuouslyin progress, new technical evolutions in 3GPP are required to securecompetitiveness in the future.

An orthogonal frequency division multiplexing (OFDM) system capable ofreducing inter-symbol interference with a low complexity is taken intoconsideration as one of next generation (after the third generation)systems. In the OFDM, a data stream is transmitted by being carried on aplurality of subcarriers. The subcarriers maintain orthogonality in afrequency dimension. Each orthogonal subcarrier experiences independentfrequency selective fading. Inter-symbol interference can be removed byusing a cyclic prefix (CP).

Orthogonal frequency division multiple access (OFDMA) is a multipleaccess scheme in which multiple access is achieved by independentlyproviding some of available subcarriers to a plurality of users. In theOFDMA, frequency resources (i.e., subcarriers) are provided to therespective users, and thus the respective frequency resources areindependently provided to the plurality of users.

A user equipment (UE) generally performs a random access procedure toaccess to a network. The random access procedure is performed to adjustuplink synchronization or to request an uplink radio resourceassignment. For one example, the UE may perform the random accessprocedure to acquire uplink synchronization after adjusting downlinksynchronization when power is initially turned on. For another example,in a state where a radio resource control

(RRC) connection is not established, the UE may perform the randomaccess procedure so that uplink radio resources are allocated for uplinktransmission. For another example, the UE may perform the random accessprocedure so that initial access to a target base station (BS) isachieved in a handover procedure.

Since the random access procedure is an initialization procedure foruplink transmission or for network access, delay or failure in therandom access procedure causes a service delay. Accordingly, there is aneed for a method capable of performing the random access procedure in amore rapid and reliable manner.

SUMMARY

The present invention provides a method of performing a reliable randomaccess procedure in a wireless communication system.

The present invention also provides a method which enables reliable datatransmission.

In an aspect, a method of performing a random access procedure in awireless communication system carried out in a user equipment isprovided. The method includes transmitting a random access preamble,receiving a random access response as a response of the random accesspreamble, wherein the random access response comprises an uplinkresource assignment and a request for transmission of a Channel QualityIndicator (CQI), and transmitting the CQI in the uplink resourceassignment.

In some embodiments, the random access response may be transmitted on aPhysical Downlink Shared Channel (PDSCH). The PDCCH may be indicated bya Physical Downlink Control Channel (PDCCH) addressed by a RandomAccess-Radio Network Temporary Identifier (RA-RNTI). The random accessresponse may be a Medium Access Control (MAC) Protocol Data Unit (PDU).The CQI may be transmitted on a Physical Uplink Shared Channel (PUSCH)and may be time first mapped in the uplink resource assignment. Therandom access preamble may be a dedicated random access preamble.

In another aspect, a user equipment includes a Radio Frequency (RF) unitfor transmitting and receiving radio signals, and a processor coupledwith the RF unit and configured to transmit a random access preamble,receive a random access response as a response of the random accesspreamble, wherein the random access response comprises an uplinkresource assignment and a request for transmission of a CQI, andtransmit the CQI in the uplink resource assignment.

In still another aspect, a method of performing a random accessprocedure in a wireless communication system carried out in a basestation is provided. The method includes receiving a random accesspreamble, and transmitting a random access response as a response of therandom access preamble, wherein the random access response comprises anuplink resource assignment and a request for transmission of a CQI.

In still another aspect, a method of performing a random accessprocedure in a wireless communication system carried out in a userequipment is provided. The method includes transmitting a random accesspreamble in a random access resource, and receiving a random accessresponse on a PDSCH indicated by a physical downlink control channel(PDCCH), wherein a cyclic redundancy check (CRC) in the PDCCH is maskedwith a random access identifier which is associated with the randomaccess resource.

In some embodiments, the random access identifier may be a RandomAccess-Radio Network Temporary Identifier (RA-RNTI). The size of theRA-RNTI may be 16 bits. The random access response may comprise a randomaccess preamble identifier corresponding to the random access preamble.The method may further include determining the random access identifierby using a subframe index of a subframe for the random access resourceand a resource index of the random access resource in the subframe. Asubframe for the PDCCH may be subsequent to a subframe for the randomaccess resource. The random access response comprises an uplink resourceassignment.

In still another aspect, a user equipment includes a RF unit fortransmitting and receiving a radio signal, and a processor coupled withthe RF unit and configured to transmit a random access preamble in arandom access resource, monitor at least one PDCCH to find a randomaccess response, and receive the random access response on a PDSCHindicated by a PDCCH when no CRC error of the PDCCH is detected, whereina CRC in the PDCCH is masked with a random access identifier which isassociated with the random access resource.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a wireless communication system.

FIG. 2 is a diagram showing functional split between an evolveduniversal terrestrial radio access network (E-UTRAN) and an evolvedpacket core (EPC).

FIG. 3 is a block diagram showing constitutional elements of a userequipment.

FIG. 4 is a diagram showing a radio protocol architecture for a userplane.

FIG. 5 is a diagram showing a radio protocol architecture for a controlplane.

FIG. 6 shows an example of frequency selective scheduling in orthogonalfrequency division multiple access (OFDMA).

FIG. 7 is a block diagram showing a transmitter using a single carrierfrequency division multiple access (SC-FDMA) scheme.

FIG. 8 is a block diagram showing a signal generator using an SC-FDMAscheme.

FIG. 9 shows a structure of a radio frame in a third generationpartnership project (3GPP) long term evolution (LTE).

FIG. 10 shows an exemplary diagram showing a resource grid for oneuplink slot.

FIG. 11 shows a structure of an uplink subframe.

FIG. 12 is a flow diagram showing a random access procedure according toan embodiment of the present invention.

FIG. 13 shows an example of transmitting a channel quality indicator(CQI) on a physical uplink shared channel (PUSCH).

FIG. 14 shows another example of transmitting a CQI on a PUSCH.

FIG. 15 shows another example of transmitting a CQI on a PUSCH.

FIG. 16 shows another example of transmitting a CQI on a PUSCH.

FIG. 17 shows an example of transmitting a CQI in a medium accesscontrol (MAC) layer.

FIG. 18 is a flow diagram showing a method of performing a handoveraccording to an embodiment of the present invention.

FIG. 19 is a flow diagram showing a method of performing a random accessprocedure according to an embodiment of the present invention.

FIG. 20 is an exemplary diagram showing transmission of a random accesspreamble and a sounding reference signal (SRS).

FIG. 21 is a flow diagram showing a method of performing a random accessprocedure according to another embodiment of the present invention.

FIG. 22 is an exemplary diagram showing transmission of an SRS in asubframe.

FIG. 23 is an exemplary diagram showing transmission of downlink data ina 3GPP LTE.

FIG. 24 is an exemplary diagram showing a method of determining a randomaccess-radio network temporary identifier (RA-RNTI) according to anembodiment of the present invention.

FIG. 25 is an exemplary diagram showing a method of determining anRA-RNTI according to another embodiment of the present invention.

FIG. 26 is an exemplary diagram showing a method of determining anRA-RNTI according to another embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc.The UTRA is a part of a universal mobile telecommunication system(UMTS). Third generation partnership project (3GPP) long term evolution(LTE) is a part of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPPLTE uses the OFDMA in downlink and uses the SC-FDMA in uplink.LTE-advance (LTE-A) is an evolution of the 3GPP LTE.

For clarity, the following description will focus on the 3GPP LTE/LTE-A.However, technical features of the present invention are not limitedthereto.

FIG. 1 shows a structure of a wireless communication system. Thewireless communication system may have a network structure of anevolved-universal mobile telecommunications system (E-UMTS). The E-UMTSmay be also referred to as a long term evolution (LTE) system. Thewireless communication system can be widely deployed to provide avariety of communication services, such as voices, packet data, etc.

Referring to FIG. 1, an evolved-UMTS terrestrial radio access network(E-UTRAN) includes at least one base station (BS) 20 which provides acontrol plane and a user plane.

A user equipment (UE) 10 may be fixed or mobile, and may be referred toas another terminology, such as a mobile station (MS), a user terminal(UT), a subscriber station (SS), a wireless device, etc. The BS 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc. There are one ormore cells within the coverage of the BS 20. Interfaces for transmittinguser traffic or control traffic may be used between the BSs 20.Hereinafter, a downlink is defined as a communication link from the BS20 to the UE 10, and an uplink is defined as a communication link fromthe UE 10 to the BS 20.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC), more specifically, to a mobility management entity (MME)/servinggateway (S-GW) 30. The S1 interface supports a many-to-many relationbetween the BS 20 and the MME/S-GW 30.

FIG. 2 is a diagram showing functional split between the E-UTRAN and theEPC.

Referring to FIG. 2, slashed boxes depict radio protocol layers andwhite boxes depict functional entities of the control plane.

The BS performs the following functions: (1) functions for radioresource management (RRM) such as radio bearer control, radio admissioncontrol, connection mobility control, and dynamic allocation ofresources to the UE; (2) Internet protocol (IP) header compression andencryption of user data streams; (3) routing of user plane data to theS-GW; (4) scheduling and transmission of paging messages; (5) schedulingand transmission of broadcast information; and (6) measurement andmeasurement reporting configuration for mobility and scheduling.

The MME performs the following functions: (1) distribution of pagingmessages to BSs; (2) security control; (3) idle state mobility control;(4) system architecture evolution (SAE) bearer control; and (5)ciphering and integrity protection of non-access stratum (NAS)signaling.

The S-GW performs the following functions: (1) termination of user planepacket for paging; and (2) user plane switching for the support of UEmobility.

FIG. 3 is a block diagram showing constitutional elements of the UE. AUE 50 includes a processor 51, a memory 52, a radio frequency (RF) unit53, a display unit 54, and a user interface unit 55. Layers of the radiointerface protocol are implemented in the processor 51. The processor 51provides the control plane and the user plane. The following methods canbe implemented in the processor 51. The memory 52 is coupled to theprocessor 51 and stores various parameters to perform a random accessprocedure and handover. The display unit 54 displays a variety ofinformation of the UE 50 and may use a well-known element such as aliquid crystal display (LCD), an organic light emitting diode (OLED),etc. The user interface unit 55 can be configured with a combination ofwell-known user interfaces such as a keypad, a touch screen, etc. The RFunit 53 is coupled to the processor 51 and transmits and/or receivesradio signals.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. A physical layer, or simply a PHY layer, belongs to the firstlayer and provides an information transfer service through a physicalchannel. A radio resource control (RRC) layer belongs to the third layerand serves to control radio resources between the UE and the network.The UE and the network exchange RRC messages via the RRC layer.

FIG. 4 is a diagram showing a radio protocol architecture for the userplane. FIG. 5 is a diagram showing a radio protocol architecture for thecontrol plane. They illustrate the architecture of a radio interfaceprotocol between the UE and the E-UTRAN. The user plane is a protocolstack for user data transmission. The control plane is a protocol stackfor control signal transmission.

Referring to FIGS. 4 and 5, a PHY layer belongs to the first layer andprovides an upper layer with an information transfer service through aphysical channel. The PHY layer is coupled with a medium access control(MAC) layer, i.e., an upper layer of the PHY layer, through a transportchannel. Data is transferred between the MAC layer and the PHY layerthrough the transport channel. Between different PHY layers (i.e., a PHYlayer of a transmitter and a PHY layer of a receiver), data istransferred through the physical channel. In the PHY layer, modulationis performed using an orthogonal frequency division multiplexing (OFDM)scheme and time and frequency can be utilized as a radio resource.

The MAC layer belongs to the second layer and provides services to aradio link control (RLC) layer, i.e., an upper layer of the MAC layer,through a logical channel. The RLC layer in the second layer supportsreliable data transfer. There are three operating modes in the RLClayer, that is, a transparent mode (TM), an unacknowledged mode (UM),and an acknowledged mode (AM) according to a data transfer method. An AMRLC provides bidirectional data transmission services and supportsretransmission when the transfer of the RLC protocol data unit (PDU)fails.

A packet data convergence protocol (PDCP) belonging to the second layerperforms header compression function. When transmitting an Internetprotocol (IP) packet such as an IPv4 packet or an IPv6 packet, theheader of the IP packet may contain relatively large and unnecessarycontrol information. The PDCP layer reduces the header size of the IPpacket so as to efficiently transmit the IP packet.

A radio resource control (RRC) layer belongs to the third layer and isdefined only in the control plane. The RRC layer serves to control thelogical channel, the transport channel, and the physical channel inassociation with configuration, reconfiguration and release of radiobearers (RBs). An RB is a service provided by the second layer for datatransmission between the UE and the E-UTRAN. When an RRC connection isestablished between an RRC layer of the UE and an RRC layer of thenetwork, it is called that the UE is in an RRC connected mode. When theRRC connection is not established yet, it is called that the UE is in anRRC idle mode.

A non-access stratum (NAS) layer belongs to an upper layer of the RRClayer and serves to perform session management, mobility management, orthe like.

Data is transmitted from the network to the UE through a downlinktransport channel. Examples of the downlink transport channel include abroadcast channel (BCH) for transmitting system information and adownlink-shared channel (DL-SCH) for transmitting user traffic orcontrol messages. User traffic of downlink multicast or broadcastservice or control messages can be transmitted on the DL-SCH or adownlink multicast channel (MCH). Data is transmitted from the UE to thenetwork through an uplink transport channel. Examples of the uplinktransport channel include a random access channel (RACH) fortransmitting an initial control message and an uplink-shared channel(UL-SCH) for transmitting user traffic or control message.

The BS manages radio resources of one or more cells. One cell isconfigured to have one of bandwidths selected from 1.25, 2.5, 5, 10, and20 megahertz (MHz) and provides downlink or uplink transmission servicesto a plurality of UEs. In this case, different cells can be configuredto provide different bandwidths. Cell configuration can be achieved insuch as manner that multiple cells geographically overlap by usingdifferent frequencies. The BS informs the UE of basic information fornetwork access by using system information. The system informationincludes necessary information which needs to be known to the UE so asto access to the BS. Therefore, the UE has to completely receive thesystem information before accessing to the BS and always has to maintainlatest system information. Since the system information has to be knownto all UEs within one cell, the BS periodically transmits the systeminformation.

Examples of logical channels mapped onto the transport channels includea broadcast channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH), a multicasttraffic channel (MTCH), a dedicated control channel (DCCH), etc.

FIG. 6 shows an example of frequency selective scheduling in OFDMA. Mostsuitable frequency bands are allocated to UEs A to G in a wholefrequency band. The size of each band or the number of bands may differaccording to a channel condition between a UE and a BS. The BS schedulesthe UEs by receiving channel information (e.g., a channel qualityindicator (CQI)) from each UE.

FIG. 7 is a block diagram showing a transmitter using an SC-FDMA scheme.

Referring to FIG. 7, a transmitter 100 includes a data processor 110, aphysical resource mapper 140, and a signal generator 150. The dataprocessor 110 processes user data and a CQI to generate complex-valuedsymbols. Functions of a MAC layer or an RRC layer in addition to aphysical layer can be implemented by the data processor 110. Functionsof the physical layer or other layers can be implemented by anadditional processor.

The physical resource mapper 140 maps the complex-valued symbols ontophysical resources. The physical resources may be resource elements orsubcarriers. The signal generator 150 generates time-domain signals tobe transmitted through a transmit antenna 190. The signal generator 150may generate the time-domain signals by using the SC-FDMA scheme. Thetime-domain signal output from the signal generator 150 is referred toas an SC-FDMA symbol or an OFDMA symbol

Although it will be assumed hereinafter that the signal generator 150uses the SC-FDMA scheme, this is for exemplary purposes only. Thus, thepresent invention may also apply to other multiple-access schemes. Forexample, the present invention may apply to various multiple-accessschemes such as OFDMA, code division multiple access (CDMA), timedivision multiple access (TDMA), and frequency division multiple access(FDMA).

FIG. 8 is a block diagram showing a signal generator using an SC-FDMAscheme.

Referring to FIG. 8, a signal generator 200 includes a discrete Fouriertransform (DFT) unit 210 that performs a DFT, a subcarrier mapper 230,and an inverse fast Fourier transform (IFFT) unit 240 that performs anIFFT. The DFT unit 210 performs the DFT on input data and thus outputsfrequency-domain symbols. The subcarrier mapper 230 maps thefrequency-domain symbols onto respective subcarriers. The IFFT unit 230performs the IFFT on input symbols and thus outputs time-domain signals.

FIG. 9 shows a structure of a radio frame in a 3GPP LTE.

Referring to FIG. 9, a radio frame includes 10 subframes. One subframeincludes two slots. A time for transmitting one subframe is defined as atransmission time interval (TTI). For example, one subframe may have alength of 1 millisecond (ms), and one slot may have a length of 0.5 ms.One slot includes a plurality of SC-FDMA symbols in a time domain and aplurality of resource blocks in a frequency domain.

The structure of the radio frame is shown for exemplary purposes only.Thus, the number of subframes included in the radio frame or the numberof slots included in the subframe or the number of SC-FDMA symbolsincluded in the slot may change variously.

FIG. 10 shows an exemplary diagram showing a resource grid for oneuplink slot.

Referring to FIG. 10, an uplink slot includes a plurality of SC-FDMAsymbols in a time domain and a plurality of resource blocks in afrequency domain. Although it is described herein that one uplink slotincludes 7 OFDM symbols and one resource block includes 12 subcarriers,this is for exemplary purposes only, and thus the present invention isnot limited thereto.

Elements on the resource grid are referred to as resource elements. Oneresource block includes 12×7 resource elements. The number N^(UL) ofresource blocks included in the uplink slot depends on an uplinktransmission bandwidth determined in a cell.

FIG. 11 shows a structure of an uplink subframe.

Referring to FIG. 11, the uplink subframe is divided into a regionassigned to a physical uplink control channel (PUCCH) for carryinguplink control information and a region assigned to a physical uplinkshared channel (PUSCH) for carrying user data. The region assigned tothe PUCCH is referred to as a control region. The region assigned to thePUSCH is referred to as a data region. A middle portion of the subframeis assigned to the PUSCH. Both sides of the data region are assigned tothe PUCCH. To maintain a single carrier property, one UE does notsimultaneously transmit the PUCCH and the PUSCH.

The PUSCH is mapped with an uplink shared channel (UL-SCH) that is atransport channel, and carries user data and/or uplink controlinformation.

Examples of the uplink control information transmitted on the PUCCHinclude an acknowledgment (ACK)/not-acknowledgement (NACK) signal usedto perform hybrid automatic repeat request (HARQ), a channel qualityindicator (CQI) indicating a downlink channel condition, a schedulingrequest signal used to request an uplink radio resource assignment, etc.The uplink control information can be transmitted on the PUCCH or thePUSCH.

The PUCCH for one UE uses one resource block which occupies a differentfrequency in each of two slots in the subframe. The two slots usedifferent resource blocks (or subcarriers) in the subframe. This is saidthat the two resource blocks assigned to the PUCCH are frequency-hoppedin a slot boundary. It is assumed herein that the PUCCH is assigned tothe subframe for 4 UEs respectively in association with a PUCCH (m=0), aPUCCH (m=1), a PUCCH (m=2), and a PUCCH (m=3).

Hereinafter, a random access procedure will be described. The randomaccess procedure is used when a UE acquires uplink synchronization witha BS or when an uplink radio resource is allocated to the UE. Afterpower is turned on, the UE obtains downlink synchronization with aninitial cell and receives system information. From the systeminformation, the UE obtains a set of available random access preamblesand information regarding resources used to transmit the random accesspreambles. The UE transmits a random access preamble randomly selectedfrom the set of random access preambles. Upon receiving the randomaccess preamble, the BS transmits a timing alignment (TA) value foruplink synchronization to the UE through a random access response.Accordingly, the UE obtains uplink synchronization.

<Random Access Procedure and CQI>

FIG. 12 is a flow diagram showing a random access procedure according toan embodiment of the present invention. The random access procedure canbe performed when a UE acquires uplink synchronization with a BS or whenthe UE acquires an uplink radio resource.

Referring to FIG. 12, the UE first acquires downlink synchronizationwith the BS (step S700). The UE acquires downlink synchronization byusing a primary synchronization signal and a secondary synchronizationsignal which are periodically transmitted by the BS.

The UE transmits a random access preamble to the BS by using a randomaccess resource (step S710). The random access preamble may randomly beselected from a random access set which includes a plurality ofavailable random access preambles. The random access set may bedetermined by using information received as a part of systeminformation. Alternatively, the random access preamble may be predefinedfor the UE. It means that the UE transmit a dedicated random accesspreamble so that no contention occurs.

Upon receiving the random access preamble, the BS transmits a randomaccess response to the UE through a downlink shared channel (DL-SCH)(step S720). The random access response may include a time alignment(TA) value for alignment of uplink time synchronization, an uplink radioresource assignment, a random access preamble identifier correspondingto the random access preamble, a cell-radio network temporary identifier(C-RNTI), etc. The TA value is used by the UE to adjust uplinksynchronization. The random access preamble identifier is an identifierfor the random access preamble received by the BS. The PDSCH whichmapped to the DL-SCH is indicated by a PDCCH addressed by a randomaccess identifier (i.e., a random access-radio network temporaryidentifier (RA-RNTI)). The RA-RNTI is masked with a cyclic redundancycheck (CRC) of control information which is carried by a PDCCH. The UEmonitors a set of PDCCH candidates. When no CRC error is detected afterPDCCH decoding is performed, the UE receives PDSCH indicated by thedetected PDCCH. In addition, the random access response may include arequest for transmission of CQI and the radio resource assignment isused for the CQI transmission.

The UE transmits a scheduled message to the BS on a PUSCH by using theuplink radio resource assignment included in the random access response(step S730). When the UE transmits the scheduled message, the UE alsotransmits a CQI which represents a downlink channel condition. The UEcan determine the CQI by receiving a downlink channel (e.g., a broadcastchannel) since the UE has already obtained downlink synchronization. Thescheduled message is a message transmitted by using the uplink radioresource assignment included in the random access response. For example,the scheduled message is a connection request message to establish RRCconnection.

After receiving the scheduled message, the BS may transmit a contentionresolution message. The contention resolution message is message toresolve contention between UEs when the random access procedure isinitiated by using randomly selected random access preamble. When the UEsuccessfully receives the contention resolution message, contention isresolved and thus an RRC connection is established. Then, the randomaccess procedure is completed.

Although the random access preamble is randomly selected from the randomaccess set, a plurality of UE can simultaneously transmit the samerandom access preambles by using the same random access resources. Thisis called contention. In practice, the BS and each UE cannot detectoccurrence of contention. After successfully receiving the contentionresolution message, the UE can know that contention is resolved and thusthe UE successfully accesses to the BS. If the UE does not receive thecontention resolution message during a predetermined time period, the UEtransmits a new random access preamble by regarding a random access asfailure. Therefore, the contention resolution message must besuccessfully received to rapidly complete the random access procedure.

By transmission of the CQI, the BS can know downlink channel conditionbefore transmitting the contention resolution message. Therefore,scheduling can be performed in a most suitable manner to successfullytransmit the contention resolution message, and a reception failure rateof the contention resolution message can be decreased.

Although the CQI is transmitted together with the scheduled message s anexample herein, the CQI may be transmitted independently from thescheduled message. In addition, the CQI may be transmitted at least oneor more times periodically or non-periodically before the BS transmitsthe contention resolution message.

Various methods can be used to transmit the CQI. The CQI may betransmitted through a physical channel (i.e., PUSCH) by being configuredin a physical layer, or may be transmitted through a UL-SCH by beingconfigured in a MAC layer.

FIG. 13 shows an example of transmitting a CQI on a PUSCH. On a subframeconsisting of 14 SC-FDMA symbols, a 4^(th) SC-FDMA symbol and an 11^(th)SC-FDMA symbol are allocated with reference signals. CQIs are allocatedto resource elements on SC-FDMA symbols adjacent to the referencesignals. By allowing the CQIs to be arranged near the reference signals,reliability of CQI transmission can be increased. The CQIs can beequidistantly distributed in a time domain and a frequency domain.

An interval or the number of resource elements to be allocated maydiffer according to a CQI amount, but the present invention is notlimited thereto. In addition, the CQIs can be arranged in time-firstmapping or frequency-first mapping. The frequency-first mapping meansthat the CQIs are arranged on resource elements allocated first alongthe frequency domain on one SC-FDMA symbol and thereafter, if resourceelements are insufficiently, the CQIs are arranged on a next SC-FDMAsymbol.

FIG. 14 shows another example of transmitting a CQI on a PUSCH.Sequences which represent CQI are time-first mapped in a subframe. Thatis, the CQIs are firstly mapped to SC-FDMA symbols at same subcarrier.For example, it is supposed that both the CQI and a transport block aretransmitted using one resource block. The transport block may carry thescheduled message. A sub-frame includes fourteen SC-FDMA symbols and twoof the fourteen SC-FDMA symbols are used as references signals.Modulation symbols which represents the CQI are mapped one by one basedon the SC-FDMA symbols. After CQI mapping is finished, the transportblock is mapped to remained resource elements. Accordingly, the CQI andthe transport block are multiplexed in a subframe. The multiplexedinformation is transmitted in the PUSCH. This can be called thescheduled message is transmitted along with CQI.

FIG. 15 shows another example of transmitting a CQI on a PUSCH. Unlikethe embodiment of FIG. 13 or 14, CQIs are transmitted throughout tworesource blocks. The CQIs can be arranged with a much larger interval toreduce data loss in a frequency domain and a time domain. By doing so, afrequency diversity can be obtained. The interval of resource elementsallocated with the CQIs may differ depending on the number of resourceblocks to be allocated and an amount of CQI information.

FIG. 16 shows another example of transmitting a CQI on a PUSCH. CQIs aredistributed on two resource blocks in a time-first mapping.

FIG. 17 shows an example of transmitting a CQI in a MAC layer. Totransmit the CQI on a PUSCH, data for a resource element allocated withthe CQI has to be punctured. In this case, partial data loss may occur,and thus the CQI may be configured as a part of a MAC protocol data unit(PDU) in the MAC layer.

Referring to FIG. 17, a MAC PDU includes a MAC header and a CQI. The MACPDU may further include a MAC service data unit (SDU) and a MAC controlelement. The MAC SDU is a data block delivered from an upper layer ofthe MAC layer. The MAC control element is used to deliver controlinformation of the MAC layer such as a buffer status report. Althoughthe CQI is appended to a part of the MAC PDU, the CQI may be located inanother position.

To indicate whether the CQI is included in the MAC PDU, a subheader ofthe MAC header includes a CQI length. The MAC header is divided into atleast one subheader. The subheader represents a length and property ofthe MAC SDU and each MAC control element. Whether the CQI is included ornot can be reported using the subheader of the MAC header, and the CQIcan be included in the MAC PDU when transmitted.

FIG. 18 is a flow diagram showing a method of performing a handoveraccording to an embodiment of the present invention. When the UE ismoving away from a serving BS, while approaching a new BS, there is aneed to perform a process of changing an access point of the UE to thenew BS over a network. The serving BS is referred to as a source BS. Thenew BS is referred to as a target BS. The process of changing the accesspoint from the source BS to the target BS is referred to as handover.

Referring to FIG. 18, a UE sends a measurement report to a source BS(step S750). The source BS decides handover (HO) according to themeasurement report (step S752). The source BS sends a handover requestmessage to a target BS (step S754). The target BS sends a handoverrequest ack message to confirm handover (step S756). The handoverrequest ack message to confirm handover may include information on adedicated random access preamble.

The source BS transmits a handover command message to the US (stepS760). The handover command message may include information on thededicated random access preamble. The US transmits the dedicated randomaccess preamble to the target BS (step S762). The target BS transmits arandom access response as a response of the dedicated random accesspreamble (step S764). The random access response may include an uplinkradio resource assignment and a request for transmission of CQI. The UEtransmits a handover confirm message with the CQI (step S766). The CQImay be multiplexed with the handover confirm message in a PUSCH.

By using the dedicated random access preamble, no contention occurs. Itis possible to perform random access procedure quickly. Accordingly,transmission delay due to handover can be minimized. Also, CQItransmission during random access procedure enables the target BS toperform downlink scheduling just after handover is completed. Thedownlink scheduling by using the CQI may improve reliability of datatransmission.

By transmission of CQI during a random access procedure, efficientdownlink scheduling can be achieving just after the random accessprocedure is completed.

<Random Access Procedure and Sounding Reference Signal>

FIG. 19 is a flow diagram showing a method of performing a random accessprocedure according to an embodiment of the present invention.

Referring to FIG. 19, a UE transmits a random access preamble to a BS byusing a random access resource (step S810). The random access preamblemay randomly be selected from a random access set. The UE transmits asounding reference signal (SRS) to the BS simultaneously with orindependently from the random access preamble (step S820). The SRS is areference signal for uplink scheduling. The BS transmits a random accessresponse including an uplink radio resource assignment in response tothe random access preamble (step S830). The random access response istransmitted through a physical downlink shared channel (PDSCH). ThePDSCH is indicated by a physical downlink control channel (PDCCH)indicated by an RA-RNTI. The BS can estimate an uplink channel conditionby using the SRS. Thus, the BS can schedule uplink radio resourcesincluded in the random access response by considering the uplink channelcondition. This means that the BS schedules the uplink radio resourcesduring the random access procedure.

The UE receives the random access response and then transmits ascheduled message according to the radio resource assignment included inthe random access response (step S840). After the BS receives thescheduled message which represents a connection request message, the BSmay transmit a contention resolution message to the UE.

FIG. 20 is an exemplary diagram showing transmission of a random accesspreamble and an SRS. It is shown that the random access preamble and theSRS are simultaneously transmitted in one subframe. The random accesspreamble is transmitted through a cyclic prefix (CP) duration T_(CP),and a preamble duration T_(PRE) in which a preamble sequence istransmitted. The CP duration T_(CP) is a duration in which a CP isinserted to minimize interference caused by a multi-path channel,inter-symbol interference, etc. The preamble duration T_(PRE) is aduration in which a sequence of random access preambles is carried. Thepreamble duration T_(PRE) may be followed by a guard interval T_(GT).For example, if a transmission time interval (TTI) of a subframe is 1.0ms, the guard time T_(GT) may be 0.2 ms. The SRS can be transmittedwithin the guard time T_(GT). Although the guard time T_(GT) istemporally posterior to the preamble duration T_(PRE) herein, the guardtime T_(GT) may be temporally prior to the random access preamble.

If the UE transmits the SRS together with transmission of the randomaccess preamble, the BS can know an uplink channel condition by usingthe SRS. The BS can allocate a band having a good channel condition tothe UE when allocating an uplink radio resource for an RRC connectionrequest message of the UE or when allocating a radio resource to the UEto which an RRC connection is established. In particular, radio resourcescheduling can be further effectively achieved in an environment where achannel condition changes significantly depending on a frequency band.

FIG. 21 is a flow diagram showing a method of performing a random accessprocedure according to another embodiment of the present invention.

Referring to FIG. 21, a UE transmits a random access preamble to a BS byusing a random access resource (step S910). The BS transmits a randomaccess response in response to the random access preamble (step S920).The random access response includes a TA value, an uplink radio resourceassignment, etc. The UE receives the random access response and thentransmits a scheduled message by using the radio resource assignmentincluded in the random access response (step S930). Further, the UEtransmits an SRS simultaneously with or independently from the randomaccess preamble (step S940). The SRS may be transmitted at a lastSC-FDMA symbol of a subframe. The SRS transmitted together with thescheduled message can be used for the purpose of scheduling for theuplink radio resource assignment after an RRC connection is establishedbetween the UE and the BS. FIG. 22 is an exemplary diagram showingtransmission of an SRS in a subframe. The SRS may be transmitted at alast SC-FDMA symbol of the subframe. The SRS may be transmitted througha data region other than a control region. When one slot includes 7SC-FDMA symbols, a demodulation reference signal (DM RS) for datademodulation is transmitted at a 4^(th) SC-FDMA symbol of the slot on aPUSCH.

Although it has been described herein that the SRS is mapped to the dataregion, that is, the last OFDM symbol, the SRS can be mapped throughoutthe data region and the control region.

After an RRC connection is established, the UE transmits the SRS to theBS periodically or in an event-driven manner so that the BS can know anuplink channel condition. However, until the RRC connection isestablished, the BS allocates an uplink radio resource to the UE withoutknowing the uplink channel condition. If the UE transmits the SRS beforethe RRC connection is established during a random access procedure, theBS can schedule initial uplink radio resources to be allocated to the UEby considering the uplink channel condition. The BS can perform furtherreliable radio resource scheduling for a UL-SCH.

<Random Access Procedure and RA-RNTI>

FIG. 23 is an exemplary diagram showing transmission of downlink data ina 3GPP LTE. A BS transmits general user data on a physical channel(i.e., PDSCH) mapped to a transport channel (i.e., DL-SCH). A PDCCHincludes downlink radio resource assignment information regarding thePDSCH. A UE first obtains control information on the PDCCH, and thendetermines how to receive and decode the PDSCH in a correspondingsubframe from the radio resource assignment information included in thecontrol information. To configure the PDCCH, the BS first determines aPDCCH format according to the control information to be transmitted tothe UE, and appends a cyclic redundancy check (CRC) to the controlinformation. The CRC is masked with a unique identifier (referred to asa radio network temporary identifier (RNTI)) according to a usage orowner of the PDCCH. The identifier has a size of 16 bits in general, andcan express from 0 to 65536. If the PDCCH is for a specific UE, a uniqueidentifier (e.g., cell-RNTI (C-RNTI)) of the UE can be masked on theCRC. If the PDCCH is for paging information, a paging indicationidentifier (e.g., paging indication-RNTI (PI-RNTI)) can be masked on theCRC. If the

PDCCH is for system information, a system information identifier (e.g.,system information-RNTI (SI-RNTI)) can be masked on the CRC. To indicatea random access response that is a response for transmission of therandom access preamble of the UE, a random access-RNTI (RA-RNTI) can bemasked on the CRC.

For example, assume that the PDCCH has a CRC masked with a C-RNTI ‘A’and transmits a random access response on the PDSCH. When the UEperforms the random access procedure, the UE transmits a random accesspreamble and then monitors the PDCCH by using the RA-RNTI. Monitoring isan operation for detecting a CRC error by decoding the PDCCH. If thereis no CRC error, the UE receives the random access response ‘B’ on thePDSCH indicated by the PDCCH. If the random access response ‘B’ includesa random access preamble identifier of the UE, it means that the UEconfirms that the random access response B is a random access responseof the UE.

The UE needs to rapidly and correctly receive the random access responseof the UE so that the random access procedure operates in a rapid andreliable manner. For this, the UE effectively configures the RA-RNTIused to identify the PDCCH of the UE.

FIG. 24 is an exemplary diagram showing a method of determining anRA-RNTI according to an embodiment of the present invention.

Referring to FIG. 24, a UE can transmit a random access preamble at asubframe of a radio frame. The number of subframes can be set to aspecific number according to a bandwidth used by a BS. The subframe thatcan transmit the random access preamble may be a subframe at which arandom access resource is located. Herein, four subframes are allowed totransmit the random access preamble in one radio frame. The subframesused to transmit the random access preamble in the radio frame can bearranged with a predetermined interval. That is, the random accessresponse can be arranged with a predetermined period.

Assume that four subframes are used to transmit the random accesspreamble in the radio frame, and the random access resource is allocatedto the subframes indexed from 1 to 4. In addition, assume that therandom access resource uses one resource block (RB). The UE can transmitthe random access preamble by using the random access resource at thesubframe indexed with 1.

The RA-RNTI is related to the random access resource by which the randomaccess preamble is transmitted. For example, the RA-RNTI can correspondto a location of the random access resource transmitted by the UE. Whenthe UE transmits the random access response at the subframe indexed with1, the BS can transmit the random access response through a subframesubsequent to the subframe indexed with 1. In this case, the RA-RNTIthat identifies the PDCCH indicating the PDSCH through which the randomaccess response is transmitted may one-to-one correspond to an index ofthe random access resource in the subframe indexed with 1.

If the subframe includes N RBs (where N is an integer greater than 1,i.e., N>1), an RB index ranges from 0 to N−1. Thus, when one randomaccess response corresponds to one RB, it can be seen that N×4 randomaccess resources are present throughout four subframes. When 64 randomaccess preambles belong to a set of random access preambles usable bythe UE, the 64 random access resources can respectively correspond tothe 64 random access preambles. For example, in a system using abandwidth of a 10 MHz, one subframe generally includes 50 RBs, and foursubframes have 200 RBs. 64 RBs selected from the 200 RBs can bedesignated as random access resources corresponding to the respectiverandom access preambles. When the random access resource is expressed ina form of (subframe index, RB index), random access resourcescorresponding to random access preambles #0, #1, #2, #3, . . . , #62,#63 can be respectively expressed by (1,0), (2,0), (3,0), (4,0), . . . ,(3, N−3), (1, N−3). When the UE transmits the random access preamble #3by using the random access resource (4,0), the UE and the BS candetermine the RA-RNTI from the random access resource without additionalsignaling. This is because the UE knows the random access resourceselected by the UE itself, and the BS knows the random access resourceused by the received random access preamble. The UE can obtain theRA-RNTI to be used by the UE itself by using a subframe index and aresource index. The UE can confirm the random access response throughthe PDCCH identified by the RA-RNTI.

Since the UE and the BS determine the RA-RNTI according to a mutuallyagreed rule, additional signaling for the RA-RNTI is unnecessary.

When the UE transmits the random access preamble through the subframeindexed with 4 and then does not receive the PDCCH identified by theRA-RNTI in a subsequent subframe, the UE can transmit the random accesspreamble through the subframe indexed with 1 in a next radio frame.

Although it has been described herein that the subframe at which therandom access preamble is transmitted is consecutive to the subframe atwhich the random access response can be transmitted, the random accessresponse may be transmitted through a subframe which is delayed by apredetermined time after the random access preamble is transmitted. Theposition and number of subframes at which the random access preamble istransmitted in the radio frame are for exemplary purposes only, and thusthe present invention is not limited thereto. In a multi-cellenvironment, the position of the subframe at which the random accesspreamble is transmitted may differ between consecutive cells.

FIG. 25 is an exemplary diagram showing a method of determining anRA-RNTI according to another embodiment of the present invention.

Referring to FIG. 25, a random access resource can be divided into twogroups according to a random access preamble number. For example, if therandom access preamble number is odd, a random access preamble can betransmitted through subframes indexed from 1 to 4, and if the randomaccess preamble number is even, the random access preamble can betransmitted through subframes indexed from 6 to 9. There is norestriction on the position and number of subframes used to transmit therandom access preamble. A method of transmitting a random accesspreamble by using a subframe used to transmit the random access preamblecan be applied in various manners. Random access preambles numbered from#0 to #31 can be transmitted through the subframes indexed from 1 to 4.Random access preambles numbered from #32 to #63 can be transmittedthrough subframes indexed from 6 to 7. As such, the random accesspreambles can be transmitted in various manners.

The subframes indexed from 1 to 4may be assigned to random accessresources corresponding to random access preambles numbered with evennumbers. The subframes indexed from 6 to 9 may be assigned to randomaccess resources corresponding to random access preambles numbered withodd numbers.

The RA-RNTI may be determined according to a subframe index at which therandom access preamble is transmitted and an index of a random accessresource in that subframe. The UE can obtain the RA-RNTI by using asubframe used by the UE to transmit the random access preamble and aposition of the random access resource in that subframe. By using theRA-RNTI, the UE can receive a random access response of the UE itself.Since the random access resource used by the random access preamble isknown to both the BS and the UE, the BS and the UE can know the RA-RNTIwithout additional signaling.

FIG. 26 is an exemplary diagram showing a method of determining anRA-RNTI according to another embodiment of the present invention.

Referring to FIG. 26, a radio frame is configured such that randomaccess preambles numbered with even numbers are transmitted through asubframe indexed with 1 and random access preambles numbered with oddnumbers are transmitted through a subframe indexed with 6. A randomaccess response for the random access preambles transmitted through thesubframe indexed with 1 can be received through any one of subsequentsubframes indexed from 2 to 4. A random access response for the randomaccess preambles transmitted through the subframe indexed with 6 can bereceived through any one of subsequent subframes indexed from 7 to 9.The UE can obtain the RA-RNTI by using a subframe index at which therandom access preamble is transmitted and an index of the random accessresource in that subframe, and can receive the random access response ona PDSCH that is indicated by a PDCCH identified by the RA-RNTI.

Without additional signaling, a random access identifier which isincluded in a random access response can be determined. A failure rateof the random access procedure can be decreased. Therefore, a servicedelay can be avoided when an initial access or a handover is performed.

The present invention can be implemented with hardware, software, orcombination thereof. In hardware implementation, the present inventioncan be implemented with one of an application specific integratedcircuit (ASIC), a digital signal processor (DSP), a programmable logicdevice (PLD), a field programmable gate array (FPGA), a processor, acontroller, a microprocessor, other electronic units, and combinationthereof, which are designed to perform the aforementioned functions. Insoftware implementation, the present invention can be implemented with amodule for performing the aforementioned functions. Software is storablein a memory unit and executed by the processor. Various means widelyknown to those skilled in the art can be used as the memory unit or theprocessor.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

1-15. (canceled)
 16. A method of performing a random access procedure ina wireless communication system, the method performed by a userequipment and comprising: transmitting a random access preamble; inresponse to the random access preamble, monitoring a downlink controlchannel by using a Random Access Radio Network Temporary (RA-RNTI) toreceive a random access response comprising an uplink resourceassignment, wherein the RA-RNTI is determined by a time domain index anda frequency domain index, the time domain index indicates a subframetransmitting the random access preamble, and the frequency domain indexindicates a frequency domain resource transmitting the random accesspreamble; and in response to the random access response, transmitting ascheduled message via a Physical Uplink Shared Channel (PUSCH), whereinthe uplink resource assignment is used for scheduling of the PUSCH. 17.The method of claim 16, further comprising: determining whether totransmit a Sounding Reference Signal (SRS); and transmitting the SRS ina last symbol in a second slot of a uplink subframe, wherein the uplinksubframe consists of two slots, each comprising a number of symbols. 18.The method of claim 16, wherein the downlink control channel is aPhysical Downlink Control Channel (PDCCH).
 19. The method of claim 16,wherein the random access response is received via a downlink sharedchannel by using the RA-RNTI, which is received via a Physical DownlinkControl Channel (PDCCH).
 20. The method of claim 16, wherein thescheduled message includes a Radio Resource Control (RRC) ConnectionRequest.
 21. The method of claim 20, wherein an uplink control signal isfurther transmitted with the scheduled message in response to the randomaccess response, wherein the uplink control signal and the scheduledmessage are transmitted on a same subframe.
 22. In a wirelesscommunication system, user equipment comprising: a Radio Frequency (RF)unit for transmitting and receiving radio signals; and a processorcoupled with the RF unit and configured to: transmit a random accesspreamble; in response to the random access preamble, monitor a downlinkcontrol channel by using a Random Access Radio Network Temporary(RA-RNTI) to receive a random access response comprising an uplinkresource assignment, wherein the RA-RNTI is determined by a time domainindex and a frequency domain index, the time domain index indicates asubframe transmitting the random access preamble, and the frequencydomain index indicates a frequency domain resource transmitting therandom access preamble; and in response to the random access response,transmit a scheduled message via a Physical Uplink Shared Channel(PUSCH), wherein the uplink resource assignment is used for schedulingof the PUSCH.
 23. The user equipment of claim 22, wherein the processoris further configured to: determine whether to transmit a SoundingReference Signal (SRS); and transmit the SRS in a last symbol in asecond slot of a uplink subframe, wherein the uplink subframe consistsof two slots, each comprising a number of symbols.
 24. The userequipment of claim 22, wherein the downlink control channel is aPhysical Downlink Control Channel (PDCCH).
 25. The user equipment ofclaim 22, wherein the random access response is received via a downlinkshared channel by using the RA-RNTI, which is received via a PhysicalDownlink Control Channel (PDCCH).
 26. The user equipment of claim 22,wherein the scheduled message includes a Radio Resource Control (RRC)Connection Request.
 27. The user equipment of claim 26, wherein anuplink control signal is further transmitted with the scheduled messagein response to the random access response, wherein the uplink controlsignal and the scheduled message are transmitted on a same subframe.